Determination of the concentrations and identities of sulfur-containing compounds is an important application in analytical chemistry. The presence of sulfur compounds in crude oils, petroleum feedstocks, and petroleum products is detrimental to the processing of petroleum and can poison and destroy expensive catalysts used in these processes. Sulfur compounds can also impart undesirable tastes and odors in food, flavors, and beverages and therefore routine measurements of the levels of sulfur compounds in a wide range of foods and beverages is performed. Many of the commonly used pesticides and herbicides are sulfur containing compounds and detection of these compounds in water, air, soil and foods is important in protection of the environment and insuring the safety of the consumer. Sulfur gases are a major source of air pollution and acid rain, requiring sensitive monitoring techniques. Many biologically important compounds contain sulfur or are converted to sulfur-containing products as part of analytical measurement techniques such as amino acid sequencing using Edman protein degradation. These examples and numerous other applications highlight the need for sensitive and selective detection of sulfur containing compounds.
For some applications, measurement of the total concentration of sulfur species is sufficient, while in other applications, the identification and quantification of individual sulfur-containing compounds is required. This later application is usually performed using chromatographic techniques including gas chromatography, liquid chromatography, supercritical fluid chromatography and other separation techniques.
Measurement of total sulfur content is usually performed using x-ray fluorescence, combustion of the sample and detection of SO.sub.2 by fluorescence or radiometric techniques. The best sensitivity of these techniques are generally in the low parts per million range, while the needs of the petroleum and chemical industries are techniques that can measure total sulfur contents in the low parts per billion range.
Numerous detection systems selective for sulfur-containing compounds have been developed for use with chromatography. Representative of the prior art is the Flame Photometric Detection (FPD), the Hall Electrolytic Conductivity Detector (HECD), the Atomic Emission Detector (AED) and several chemiluminescence-based detectors. The FPD is not well suited for sulfur detection having a non-linear response for sulfur compounds and suffering from quenching of the sulfur response due to the presence of non-sulfur-containing compounds. The HECD is not widely used for sulfur detection and it is difficult to operate and maintain. Both the FPD and HECD have different responses for various sulfur-containing compounds which makes calibration of the detectors difficult and prohibits the use of these detectors for the measurement of total sulfur content of a sample. The AED offers advantages over other sulfur-selective chromatographic detectors; however, the apparatus is expensive and requires highly skilled operators.
An improved system for the measurement of sulfur compounds in air was developed by Benner and Stedman and is described in 61 Anal. Chem. 1268-71 (1989). In this system, sulfur compounds are combusted in a hydrogen-rich/air flame to produce sulfur monoxide. The combustion products are collected by means of a quartz sampling probe and transferred to a separate reaction chamber where the gases are mixed with ozone. Sulfur monoxide reacts with ozone to produce electronically excited sulfur dioxide, which relaxes by emission of light (h .nu.) in the blue and ultraviolet region of the spectrum.
Sulfur Compound.fwdarw.SO+other products EQU SO+O.sub.3 .fwdarw.SO.sub.2 *.uparw.+O.sub.2 EQU SO.sub.2 *.fwdarw.+SO.sub.2 +h.nu.
This Sulfur Chemiluminescence Detector (SCD.TM.) has a linear and equimolar response for sulfur compounds and does not suffer from interference or quenching due to the presence of non-sulfur-containing compounds.
A detection system for chromatography based on the SCD technique has been developed by Godec and Johansen and is described in U.S. application Ser. No. 07/759,105 filed Sep. 6, 1991. The chromatography detector employs a conventional flame ionization detector (FID) operated under hydrogen rich conditions as the combustion source for the production of sulfur monoxide. A high purity ceramic probe is positioned in the FID flame and used in conjunction with a vacuum pump to collect approximately 90% of the flame's gases and to transfer the sulfur monoxide to a reaction chamber. Ozone is mixed with the flame gases in the reaction chamber and the emitted light is monitored using a photomultiplier tube. This configuration permits simultaneous monitoring of the FID and SCD detector signals, providing information on both the sulfur components and non-sulfur compounds in a sample.
This SCD technology forms the basis of commercial instruments for use with gas, liquid and supercritical fluid chromatography and provides high sensitivity (&lt;5 pg S/sec), high selectivity (&gt;10.sup.6 g S/g C) with no interference or quenching of the sulfur signal from non-sulfur compounds. Despite offering improved performance for sulfur-selective detection in chromatography, the basic FID/SCD detection system has some limitations. A flame ionization detector is required for use of the SCD and the sensitivity and selectivity of the detector is strongly dependent on the flow rates of hydrogen and air to the FID and the position of the ceramic probe in the flame. Adjustment of the position of the probe is therefore required for operation and some degree of skill and training is required for this adjustment. Silicon compounds and other materials such as the bleed from a gas chromatographic column can cause loss in sensitivity of the detector by deposition of materials on and inside the ceramic probe. Finally, many applications require the detection of sulfur compounds at concentrations below the detection limit of the current SCD instrumentation. The need for improved sensitivity, ease of operation, greater utility and elimination of factors which reduce sensitivity has led to the development of an improved system for the measurement of sulfur compounds.
Improved sensitivity of the SCD should be possible. Benner and Stedman have shown using gas phase titration of SO produced in the quartz burner with NO.sub.2 that only 0.4% of the sulfur molecules entering the quartz burner are converted to SO. See 61 Anal. Chem. 1268-71 (1989). Thermodynamic studies have shown that the concentration of SO can be increased by operation of the combustion under optimum hydrogen and air mixtures. The present invention employs a new ceramic burner assembly which permits operation under conditions which increase the production of sulfur monoxide and improve the sensitivity of the SCD.